Surface relief volume reflective diffractive structure

ABSTRACT

A surface relief structure includes a recording medium configured to be structurally modified when exposed to interfering and non-interfering portions of radiation beams, the structurally modified recording medium including, when viewed in a two-dimensional cross-section along one of the axes of the recording medium a plurality of equally spaced steps of fine-sized periodicity superimposed upon a plurality of deep depressions of substantially coarse-sized periodicity. The structurally modified recording medium is configured to produce in reflection single and multiple colors in a broad spectral range when illuminated by a source of light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to,U.S. patent application Ser. No. 12/031,381, entitled “Surface ReliefVolume Reflective Diffractive Structure,” filed Feb. 14, 2008, which inturn claims priority to provisional U.S. application Ser. No.60/901,579, entitled “Volume Surface Relief Phase Reflection Holograms”,and filed Feb. 14, 2007, the contents of all of which are herebyincorporated by reference in their entireties.

BACKGROUND

The present invention relates to surface relief-volume reflectivediffractive structures, and methods and systems for producing suchstructures, and more particularly to surface relief-volume reflectivediffractive structures for easily recording replicable, single andmulticolor diffractive images, and methods and systems for producingsuch structures.

Conventional surface relief diffractive structures generally include ashallow edge profile, e.g., 100-300 nanometers, that is sinusoidal inshape and has a pitch, or spacing between maxima of the sine waveprofile, of around one micrometer. Such a shallow profile generallyenables relatively easy mechanical replication of the structure. Theshallow profile of such surface relief structures resulting from aninitial recording of an object's image, using continuous wave laserlight, in the photosensitive material diazonaphthoquinone, commonlyknown as AZ photoresist, or simply, photoresist. The recording geometrygenerally included two beams of light, in which one beam, correspondingto light from an object (i.e., “object” beam) interferes with a secondbeam (the “reference” beam). Both beams are initially incident on thesame side of the recording medium such that the resultant interferencefringes are mainly perpendicular to the surface. This exposure techniqueis referred to as an “off-axis” exposure, since both beams are at anangle with respect to the normal to the surface. Photoresist materialhas the property that when exposed and developed, the material isremoved (etched) from the top surface downwards in proportion to theexposure intensity. For a typical hologram, such exposure intensity issinusoidal in nature such that the resulting surface of photoresist hasa sine wave variation in depth. One technique to mechanically replicatethe developed photoresist, the photoresist is coated with a layer ofconductive metal and converted into a hard nickel replica throughelectroplating techniques. The hard nickel replica can then be used toimpress the surface pattern into plastic using, for example, heat andpressure, or through ultraviolet casting. The typical diffractionspectrum from a hologram formed in this way covers the realm of theentire visual range from blue to red, or about 400 to 700 nanometers(thus, the resultant holograms are referred to as “rainbow holograms”).It is not feasible to produce with shallow surface relief structuressingle, individual, color holograms.

Individual colors can be made holographically with a different type ofdiffractive device called a volume reflection hologram (or sometimesvolume phase reflection hologram). In this case, the object andreference light beams are brought into the recording medium fromopposite sides, and the interference structure that forms upondevelopment is a set of planes separated by a distance half thewavelength of the incident light divided by the index of refraction ofthe medium. The spacing in this case is very fine. For example, withgreen light having a wavelength of 500 nanometers and a recording mediumhaving an index of refraction of n=1.5, the fringe spacing isd=500/(2×1.5)=167 nanometers. For this structure a different medium thanphotoresist is generally used. Typical recording materials includedichromated gelatin (DCG), photopolymer, and silver halide. Thesediffractive planes are not surface relief structures, but rather includeregions of different indices of refraction induced by the exposure. Inreconstruction of the recorded image with incident white light, a smallportion of light is reflected from each of the interference planes, andbecause of the half-wavelength spacing, the reflected light is coherentwith reflection from all the other planes. The coherently reflectedlight is viewed as a single color that is the same as the color of theoriginal recording light. The remaining colors are incoherentlyscattered out of the field of view. Because the fringe structureincludes of a set of parallel planes distributed throughout the medium,there is no surface relief structure that can be mechanicallyreplicated. Thus, replication of these volume holograms is performedoptically, using, for example, a laser exposure on a production line.

Another technique for recording holographic images is one predicated onusing a recording geometry similar to that used for recording volumeholograms (i.e., directing the object beam and reference beams fromopposite sides of the recording medium) to record the interfering andnon-interfering patterns in a thick layer of photoresist, so that twosets of interference fringes are formed that are perpendicular to eachother. When this recording is developed, etching of the photoresistproceeds in a manner similar to the recording of thin-layer photoresistmedia due to the off-axis exposure. However, because of the additionalplanes produced from the counter propagating beams, the resultant sinewave profile becomes modified into a stepped profile, with the stepsseparated by a half-wavelength, as with the volume case. In this casethe profile has the appearance, from the edge, of a stepped pyramid(thus inspiring the name given to the structure of an AZTEC structurewhich due to the resemblance of the resultant structure to Aztec templepyramid, and also because Aztec is a useful mnemonic of the recordingtechnique diazo photoresist technology). Resultant stepped-pyramidstructures can be mechanically replicated in a manner similar to themechanical replication of shallow surface relief structures. Suchstepped-pyramid structures also produce, upon illumination by a lightsources, a single-colors in reflection (i.e., each point of thereflected image includes a single color, related to the originalrecording color, rather than a rainbow of colors produced when a shallowsurface relief structure is illuminated.

While Aztec structures formed in this way do demonstrate properties ofvolume holograms, there are some significant differences in thediffraction characteristics. Because the recording is done by twoseparate exposures, one surface and one volume, the diffracted light hasproperties of both. The stepped structure does indeed produce singlecolor reflected light. However, because it is a surface reliefstructure, it also produces rainbow reflected light as well. The singlecolor light appears primarily on-axis, while the rainbow light appearspredominantly off-axis.

Another feature of conventional holographic recording techniques andsystems is that the recording of holographic images into photoresistlayers requires use of blue or shorter wavelength light, and thus manyof the recording geometries of conventional system produced images thatare restricted to the blue end of the visible spectrum.

SUMMARY

It would therefore be desirable to produce a replicable surface reliefstructure that exhibits the single color reflective property of volumeholograms and the property of suppressing and/or modifying the off-axisrainbow colors. It would thus be desirable to provide an improved Aztectype diffractive device which accomplishes this objective.

It would also be desirable to have recording techniques and systems thatenable Aztec diffractive devices to be easily produced and which enableviewability throughout the entire visible spectrum.

The present disclosure provides significant improvement over systems,techniques and structures described in such issued patents as U.S. Pat.No. 4,888,260, U.S. Pat. No. 4,874,213, U.S. Pat. No. 4,839,250, andU.S. Pat. No. 4,999,234, the contents of all of which is herebyincorporated by reference in their entireties. These issued patentsdescribed techniques and systems to enable making mechanicallyreplicable surface relief diffractive structures that would exhibitproperties similar to those of volume diffractive structures.

More particularly, the present disclosure relates to Aztec structuresthat are configured such that the diffracted light includes a singlecolor component with the highest efficiency possible while suppressingor modifying in multicolor rainbow color components. This is achieved byaltering the recording geometry such that high efficiency skewedsymmetric and asymmetric groove profiles are produced in a recordingmedium, and also by taking advantage of resulting diffraction efficiencycharacteristics of these particular surface relief structures withrespect to those of typical volume structures. The disclosure providedherein has two central aspects. The first aspect pertains to a generaltheoretical consideration for deep structured surface relief profilesthat can be produced by a variety of techniques. The second aspectpertains to fabrication techniques utilizing holographic techniques.

To illustrate properties of the Aztec structures, reference is made toFIGS. 19 and 20. Particularly, FIG. 19 provides a comparison of thediffraction efficiencies in reflection of surface relief gratings as afunction of depth for two case: a) a surface grating having no steps, 2)the same surface grating containing well defined equally spaced stepsthat demonstrate volume effects. With respect to the first case, thestructure could be, for example, a sine wave profile for which thesurface efficiency varies as a Bessel function, reaching a maximum at acertain depth and then decreasing to zero. For a triangular profile, theefficiency varies as a cosine squared function, also rising to a maximumand then decreasing. FIG. 19B shows the diffraction efficiency as afunction of depth for the zero and the +1 and −1 orders for anon-symmetric triangular grating (shown in FIG. 19A) with a center pointdisplaced from 0.5 of the grating period to 0.65 of the period), with nosteps. To compute the diffraction efficiency, the Grating Solver™program (v. 4.20) was used to perform full vector calculation for agrating period of 3.5 microns and 500 nanometer p-polarized light atnormal incidence. As shown, the +1 and −1 reflected orders are seen torise to a maximum at relatively shallow depths and then decrease tonearly zero at large depths. Because of the skewed symmetry, the +1 and−1 orders are not equal, thus demonstrating the partial blaze for thisstructure. A significant change in the diffracted light occurs when thestep structure is added. For incident light having a wavelength of 500nanometers, a resonance is expected for a step height that is half thewavelength, or 250 nanometers, where the dielectric layer covering thesteps is taken to be air, with an index of refraction, n=1. Referring toFIG. 19D, the diffraction efficiency, as a function of depth, iscomputed for a grating having three equally spaced steps (as shown inFIG. 19C). Maximum diffraction efficiency is seen to occur when thegrating depth is equal to three steps, i.e., 750 nanometers. Referringto FIG. 19F, the diffraction efficiency, as a function of depth, iscomputed for a grating structure having a height of five (5) steps (asshown in FIG. 19E), with each step having an individual height of 250nanometer. As shown, maximum diffraction efficiency occurs when thegrating depth is equal to five steps, i.e., 1250 nanometers. Theseresults for the volume step structure effects can be compared with thegeneral results for a conventional volume structure such as silverhalide or DCG, for example, for which the efficiency varies as ahyperbolic function that slowly rises to a constant maximum value as thefringe structure reaches a maximum stage of development.

Referring to FIG. 20, the diffraction efficiency results in reflectionfor spectral scans from 300 to 800 nanometers for three cases of thegrating profiles of FIG. 19 (i.e., the profiles shown in FIGS. 19A, 19Cand 19E) are shown. FIG. 20A is a graph of the diffraction efficiency,as a function of wavelength, for a grating profile having a depth of1200 nanometers (non-symmetric profile, s-polarization, and normalincidence) without any step structure. As seen from the graph, there isno resonance behavior and practically no light is diffracted. FIG. 20Bis a graph of the diffraction efficiency, as a function of wavelength,for a grating profile having three steps, each with a height of 250nanometers, or a fixed depth of 750 nanometers. Here too,s-polarization, and normal incidence were assumed for computation of theresults. As shown, for this grating profile there is resonance thatoccurs at 500 nanometers. FIG. 20C is a graph of the diffractionefficiency, as a function of wavelength, for a grating profile havingfive (5) steps, 250 nanometers each, and a fixed depth of 1250nanometers. Again, s-polarization, and normal incidence were assumed forcomputation of the results. As shown, because more steps were used inthis profile, the resonance lobe at 500 nanometers is narrower than theresonance lobe that occurred for the profile having three steps becausethe bandwidth varies inversely to the number of steps. The structuresand results shown in FIGS. 19 and 20 are examples of generalconsiderations regarding gratings with and without step structures thatcould be formed by a variety of techniques.

Some theoretical aspects of stepped structures, using scalar diffractiontheory, are described in the paper by J. J. Cowan entitled “AztecSurface Relief Volume Diffractive Structure”, Journal of the OpticalSociety of America, Vol. 7, No. 8, August, 1990, pp. 1529-1544, thecontent of which is hereby incorporated by reference in its entirety. Inthat paper diffraction efficiencies for purely symmetric and purelyasymmetric profiles having two to ten steps were considered. It wasshown that high diffraction efficiency resulted for both the symmetricand asymmetric profiles at a particular wavelength. However, asymmetricgrating profiles enabled other wavelengths adjacent to the primary oneto have high efficiencies as well. Additionally, experiments have shownthat narrower color bands resulted from a greater number of steps in theprofile, with a bandwidth proportional to the reciprocal of the numberof steps. In the present disclosure, the theoretical aspects areextended to the more accurate full vector treatment, and calculationsare performed with the software program provided by the Grating SolverCorporation.

Photoresist material, a recording medium that can be used in conjunctionwith the recording systems and techniques described herein, has itshighest absorption in the ultraviolet range. The absorption drops offconsiderably in the visible range. Most recordings using holographictechniques for the typical surface relief hologram are thus performedwith blue to violet light. For example, the 413 nanometer line of thekrypton laser, the 441.6 nanometer line of the He—Cd laser, the 457nanometer line of the argon gas laser or diode-pumped solid state laser,and the 468 nanometer line of the krypton gas laser may all be used inhologram recording systems. Longer wavelengths than these are generallynot effective in recording wavelengths on photoresist materials. Anadditional consideration in the implementation of Aztec recordingsystems has to do with the fact that counter propagating light has to beabsorbed through a comparatively thick layer. This means that if thewavelength of the light is too short, absorption will take place in thefirst thickness layers and will be largely absorbed before it has achance to form the volume fringes necessary for the stepped profile.Another consideration is the color shift phenomenon that occurs when aholographic image recorded in the photoresist medium is replicated in aplastic medium. The fringe spacing within the photoresist is half thewavelength of the recording light divided by the index of refraction ofphotoresist, which in the visible range approaches n=1.7. For anincident wavelength of 457 nanometers, the spacing is thus457/(2×1.7)=134 nm. If this pattern is thus embossed into plastic havingan index of n=1.5 and then viewed through the plastic base, the observedwavelength is 2×134×1.5=402 nm, which is in the violet. Thus there is asignificant color shift from 457 to 402 nanometers due entirely to thedifferences in the respective indices of refraction of the mediainvolved.

These complications may be avoided by, for example, using a specialprism coupling techniques and/or using non-typical replicationmaterials. The prism coupling techniques used herein include couplingthe prism employed directly into an actual embedded (i.e., encapsulated)object and/or coupling the prism to a volume phase hologram of an actualobject. Non-typical replication materials include higher index materialsthan those conventionally used to thus minimize the color shifting.

With prism coupling, the object, whether actually encapsulated under aflat cover glass or in the form of a volume hologram recorded andembedded between flat glass plates, is first coated with a layer ofphotoresist and is then contacted to one face of a prism. The contactcan be done in a number of ways, one of them being the use of a liquidcontact layer such as xylene, which is an ideal index match for glass.Another effective contact method is the use of a melted gelatin mix.This could include, for example, one part gelatin granules mixed withfour parts of water, that is then allowed to swell, and subsequentlymelted down. Applied as a heated liquid, the gelatin solidifies at roomtemperature. After exposure, the gelatin layer can simply be strippedaway, allowing the photoresist layer to be developed. The incidentrecording laser light is incident through one of the other faces of theprism onto the object at an angle that is greater than the criticalangle of reflection, defined by sin θ=1/n, where n in this case is theindex of refraction of the prism. It is possible, using blue laserrecording light, for the light to be incident at a large angle onto theobject and to record counter propagating beams. The fringe pattern inthis case is between the incident laser light and the light reflectedfrom the object, both at large angles, such that the spacing is typicalof the hologram colors such as green and/or red, rather than the colorof the recording light. At the same time, because of the difference inindex of refraction of the photoresist layer and the glass cover ofeither the object or the hologram, a portion of the incident light isreflected in the same general direction as the light reflected from theobject, but at a small angle with respect to it. Initially, care istaken to tilt the actual object with respect to the cover glass, or ifthe object is in the form of a hologram, to have a built in tilt to theobject at an appropriate viewing angle, usually in the range of 20 to 30degrees from the normal.

The interference between the light reflected from the object and thatreflected from the glass-photoresist interface forms a set orinterference fringes that are tilted at a relatively large angle withrespect to a normal to the surface and with relatively large spacing dueto the small angle between the two beams. At the same time, the finefringe pattern resulting from the interference between the incidentlight and the light reflected from the object is a set of parallelfringes that are inclined (or skewed) at a relatively shallow angle withrespect to the surface. The resulting pattern from these two sets ofinterference fringes, upon exposure and development, is a steppedprofile that is more asymmetric than symmetric.

During the development stage, the recording system takes advantage ofthe relative diffraction efficiencies of the two types of exposure—theoff-axis and the volume. The off-axis efficiency varies roughly as aBessel function, for an approximate sine wave depth profile, where theefficiency, as a function of etch depth, rises from zero to a maximumand then decreases again to zero. The volume efficiency, on the otherhand, for a conventional non-surface-relief structure, varies with etchdepth roughly as a hyperbolic function, rising slowly to a maximum valueand then remaining at a large value for a large etch depth. For astepped surface-relief volume structure, the efficiency rises to maximumvalues as each stepped layer is uncovered by the development process.Since the unwanted rainbow spectra arise from the off-axis exposure,their suppression depends on etching deep enough until the efficiencyhas cycled through the maximum value and has reached its minimum value.At the same time, deep etching produces a maximum efficiency of thevolume exposure, which is the single color desired result. Forsurface-relief volume structures, the deeper the etching, the narrowerthe bandwidth of generated single color components.

Additional effects can arise from use of the above-discussed recordingsystem configurations. With respect to the diffraction efficiency, it ispossible, for a smaller number of steps, to have several efficiencypeaks in the visible spectrum for separate colors. For example, if thereare five steps in a grating profile, a real orthoscopic image of theobject could be viewed at the intended viewing angle. However, if thedeveloped hologram was tilted to the opposite angle from the normal, apseudoscopic image could appear in a different color from the realimage. For a larger number of steps, e.g., ten (10) to fifteen (15), itis possible to have several peaks, very closely spaced to each other,that fit under an envelope such that the observed diffracted light hasthe appearance of a single color. This has an advantage over typicalmonochromatic narrow-band volume holograms which isolate a narrowspectral band out of the whole visible spectrum. On the other hand, anenvelope that encompasses several narrow-band wavelengths enables morelight to be extracted from the visible spectrum, and thus the observedperceived single color is bright. This perceived single color can bevaried, depending on the geometry of the stepped profile. A totallysymmetric stepped profile (i.e., a profile in which the center point ofthe pyramid, the apex, is exactly halfway between minima constituting aperiod of the profile) having many steps will produce a predominantlysingle color component with high efficiency and several other colorcomponents at lower efficiency. On the other hand, a totally asymmetricstepped profile (i.e., a profile in which the top of the pyramid occursjust before the next step drops to the minimum of the next period of theprofile), having a large number of steps will produce not only apredominant single color but all adjacent colors as well, which fitunder an envelope that is typical of the diffraction character of ashallow blazed hologram. In the intermediate case (i.e., in situationsin which the grating profile has a configuration of between that of atotally symmetric and totally asymmetric configurations) the primary (orresonance) wavelength is accompanied by a few adjacent wavelengths thatare concentrated in a band that appears as the perceived single color.Furthermore, since the wavelength envelope includes several narrowbandcolors, the perceived depth of an image is also greater than it wouldhave been for a single broadband color. For a conventional volumehologram a large depth is perceived when the color is nearlymonochromatic, but at the same time the image is dim because of thenarrow bandwidth. For an Aztec volume structure, on the other hand, thedepth is perceived as large because of the narrowband nature of eachresonant wavelength, but the image is bright because there are manyclosely spaced wavelengths.

In some embodiments, recording of an object can be performed by using ahologram of an object rather than an actual object. Recording an imagein a surface relief structure using a hologram of an object rather thanrecording the actual object itself offers some advantages. Firstly, therequired built-in tilt to provide the proper viewing angle can be donein a conventional way using a volume hologram such that when the copy ismade into a recording medium (photoresist) there is no distortion of theobject due to its tilt. With an actual embedded (encapsulated) object,such distortion is generally unavoidable. Another advantage is relatedto the amount of light that can be provided for the off-axis exposure.An advantage of a prism coupling technique is the fact that if incidentlight is detuned from reconstructing the holographic image of theobject, it will be totally reflected from the interface between theprism and the air. Thus if there is a slight detuning then some lightwill be reflected from the holographic object and the remainder will betotally reflected. These two beams of light will then interfere, bothwith a small angular separation and both inclined at a small angle withrespect to the surface. By varying the amount of detuning, the relativestrength of the off-axis exposure can be adjusted with respect to thatof the volume exposure.

In some embodiments a split-beam system can be advantageously used torecord holographic images from a volume hologram. In a split-beamconfiguration, one beam is incident at the resonant angle onto thehologram of the object, and then reflected away from it. A second beamcan then be brought in, by virtue of a second prism coupling, on theopposite side of the volume hologram, at a slightly off-axis angle fromresonance. If both these beams use s-polarized light instead of thenormal p-polarized light, then only the two beams progressing at a verysmall angle away from the hologram will interfere to produce theoff-axis exposure needed for the opening set of fringes. The first beamthat is incident onto the front side of the hologram before beingreflected will not interfere with the second beam incident from the backside because their polarizations will be nearly orthogonal.

The above prism coupling procedures is similar to that described in theco-owned U.S. Pat. No. 6,608,722 and U.S. Pat. No. 6,940,665, bothentitled “Directional Diffuser”, the contents of which are herebyincorporated by reference in their entireties. The directional diffuserdescribed in the above issued patents pertains to producing shallowprofiles so that broad band light is diffracted at an angle. The prismcoupling procedure described in relation to surface relief volumestructures produces deep structured profiles so that light is diffractedin single colors with full parallax (i.e., the color produced does notchange if the hologram is tipped forward or backward or left or right,but rather stays constant regardless of how the angle of incidencevaries or how the hologram is observed)

The systems, techniques and structures described herein apply not onlyto one-dimensional gratings, i.e., for surfaces defined as the x-y planein which the step structure variation is only in a single direction(e.g., the x-direction) and is constant in another direction (e.g., they-direction), but also apply to gratings that include variations in twoor more directions (e.g., variations in both the x and y-directions).Such grating correspond, for example, to structures that may be producedthrough the introduction of a diffuse image that gives angular variationalong both the x and y directions. A general result for the most extremecase is that the strong zero order resonance can remain strong while theadjacent orders (i.e., side lobes) can be reduced in intensity.

In one aspect, a surface relief structure is disclosed. The structureincludes a recording medium configured to be structurally modified whenexposed to interfering and non-interfering portions of radiation beams,the structurally modified recording medium includes, when viewed in atwo-dimensional cross-section along one of the axes of the recordingmedium a plurality of equally spaced steps of fine-sized periodicitysuperimposed upon a plurality of deep depressions of substantiallycoarse-sized periodicity. The structurally modified recording medium isconfigured to produce in reflection single and multiple colors in abroad spectral range when illuminated by a source of light.

Embodiments of the structure may include one or more of the followingfeatures.

The radiation beams may include at least one of, for example, lightbeams and/or electron beams.

The plurality of deep depressions of coarse-sized periodicity mayinclude a grating structure varying along one direction in thetwo-dimensional cross-section view.

The period of the coarse-sized grating structure may be substantiallysymmetric when viewed in the two-dimensional cross-section view.

The period of the coarse-sized grating structure may be substantiallyasymmetric when viewed in the two-dimensional cross-section view.

The period of the coarse-sized grating structure may be skewed whenviewed in a two-dimensional cross-section view relative to a principalsurface of the recording medium prior to exposing the recording mediumto portions of the radiation beams.

The plurality of equally spaced steps of fine-sized periodicity may beapproximately half of a wavelength of illuminated light observed inreflection when the microstructure is coupled to an overcoatingdielectric having an index of refraction approximately equal to 1.

The recording medium may be overcoated with a clear dielectric having anindex of refraction, n, such that the illuminated light observed inreflection corresponds to a wavelength equaling to approximately twicethe product of n and a height value of any of the equally spaced steps.

Light diffracted, upon illumination of the structure with a lightsource, may include light having wavelengths corresponding topredominantly a single color resonance whose bandwidth varies in inverseproportion to a number equaling the plurality of steps.

The plurality of deep depressions of coarse-sized periodicity mayinclude a grating structure varying along in two directions of thetwo-dimensional cross-section view.

Light diffracted, upon illumination of the structure with a lightsource, may include multiple single-color resonances of near-equalintensities.

Light diffracted, upon illumination of the structure with a lightsource, may include a single color resonance that becomes narrower andbunched closer together such that it can fit under a broad spectralenvelope as the number of steps increases.

Light diffracted, upon illumination of the structure with a lightsource, may include a single color resonance that is accompanied byseveral adjacent resonances of lower intensity such that a perceivedsingle color is modified by the spectral composition of the adjacentcolors.

Light diffracted, upon illumination of the structure with a lightsource, may include green and blue resonances that combine to producelight corresponding to the color cyan.

Light diffracted, upon illumination of the structure with a lightsource, may include green and red resonances that combine to producelight corresponding to the color yellow.

Light diffracted, upon illumination of the structure with a lightsource, may include blue and red resonances that combine to producelight corresponding to the color magenta.

Each of the plurality of deep depressions of coarse-sized periodicitymay have a depth sufficient to diffract light that, in the absence ofsteps, would have a minimal diffraction efficiency.

The structure may further include a diffuse component such that parallaxviewability is increased over a large horizontal and vertical range ofangles.

In another aspect, a method for recording a holographic image of anobject is disclosed. The method includes providing a recording mediumconfigured to record a holographic image, the recording medium havingtwo principal surfaces and placing the object proximate one of the twoprincipal surfaces of the recording medium. The method also includesdirecting radiation beams at the other surface of the two principalsurfaces of the recording medium at an angle with respect to a normal tothe other of the two principal surfaces of the recording medium suchthat a resultant radiation beam reflected from the object interfereswith a resultant first reference beam progressing in a direction that isat least partly similar to the progression direction of the radiationbeam reflected from the object, both beams being separated by a smallangle such that a first interference fringe structure is formed in therecording medium that is skewed at an angle with respect to any of thetwo principal surfaces of the recording medium, and such that aresultant second reference beam progresses in a general direction thatis at least partly opposite to the direction of the radiation beamsreflected from the object such that a fine-sized set of interferencefringes is formed in the recording medium, the second set ofinterference fringe being skewed with respect to the any of the twoprincipal surfaces of the recording medium.

Embodiments of the method may include any of the features describedabove in relation to the structure as well as one or more of thefollowing features.

The first interference fringe structure may include at least one of, forexample, an inclined first interference fringe structure and/or anon-symmetric first interference fringe structure.

Providing the recording medium may include providing a photoresistmedium.

Placing the object may include encapsulating the object in an opticallyclear material, and placing the encapsulated object proximate the one ofthe two principal surfaces of the recording medium. Encapsulating theobject in the optically clear material may include encapsulating theobject in an optically clear cement index matched to a cover glass.

Placing the object may include placing a conventional volume hologram ofa tangible object proximate the one of the two principal surfaces of therecording medium. The conventional volume hologram may be encapsulatedbetween flat glass plates, and the conventional volume hologram may berecorded in one of, for example, dichromated gelatin (DCG), photopolymerand/or silver halide. Placing the conventional volume hologram mayinclude placing the conventional volume hologram at an angle withrespect to any of the two principal surfaces of the recording medium.

The method may further include reproducing holographic images of theobject based, at least in part, on the first and second interferencestructures formed in the recording medium.

providing the recording medium may include providing a recording mediumconfigured to be structurally modified when exposed to interfering andnon-interfering portions of radiation beams, the structurally modifiedrecording medium including, when viewed in a two-dimensionalcross-section along one of the axes of the recording medium a pluralityof equally spaced steps of fine-sized periodicity superimposed upon aplurality of deep depressions of substantially coarse-sized periodicity,the structurally modified recording medium is configured to produce inreflection single and multiple colors in a broad spectral range whenilluminated by a source of light.

In a further aspect, a system to record a holographic image isdisclosed. The system includes a recording medium having two principalsurfaces and configured to record a holographic image of an objectplaced proximate one of the two principal surfaces of the recordingmedium, and a radiation source to direct radiation beams at the othersurface of the two principal surface of the recording medium at an anglewith respect to a normal to the other of the two principal surfaces ofthe recording medium such that a resultant radiation beam reflected fromthe object interferes with a resultant first reference beam progressingin a direction that at least partly is similar to the progressiondirection of the radiation beam reflected from the object, both beamsbeing separated by a small angle such that a first interference fringestructure is formed in the recording medium that is inclined at an anglewith respect to any of the two principal surfaces of the recordingmedium, and such that a resultant second reference beam progresses in ageneral direction that at least partly is opposite to the direction ofthe radiation beams reflected from the object such that a fine-sized setof interference fringes is formed in the recording medium, the secondset of interference fringe being inclined with respect to the any of thetwo principal surfaces of the recording medium.

Embodiments of the system may include any of the features describedabove in relation to the structure and/or the method, as well as thefollowing feature.

The radiation source may be a laser light source.

In yet another aspect, a method for recording a holographic image of anobject is disclosed. The method includes providing a recording mediumconfigured to record holographic image, the recording medium having twoprincipal surfaces, placing the object proximate one of the twoprincipal surfaces of the recording medium, and coupling a prism at acontact surface of the prism to the other of the two principal surfacesof the recording medium. The method also includes directing light beamsat another surface of the prism such that a resultant radiation beamreflected from the object interferes with a resultant first referencebeam progressing in a direction that is at least partly similar to theprogression direction of the radiation beam reflected from the object,both beams being separated by a small angle such that a firstinterference fringe structure is formed in the recording medium that isskewed at an angle with respect to any of the two principal surfaces ofthe recording medium, and such that a resultant second reference beamprogresses in a general direction that is at least partly opposite tothe direction of the radiation beams reflected from the object such thata fine-sized set of interference fringes is formed in the recordingmedium, the second set of interference fringe being skewed with respectto the any of the two principal surfaces of the recording medium.

Embodiments of the method may include any of the features describedabove in relation to the structure, method and system, as well as one ormore of the following features.

Coupling the prism may include coupling an equilateral prism having anindex of refraction, n, of approximately 1.5, at a contact surface ofthe equilateral prism.

Coupling the prism may include placing an interfacing material betweenthe coupling surface of the prism and the other of two principalsurfaces of the recording medium. The interfacing material may includean index-matching liquid material that includes at least one of, forexample, xylene, decalin and/or water.

Placing the object may include placing a conventional volume hologram ofa tangible object proximate the one of the two principal surfaces of therecording medium. The method may further include coupling one surface ofa glass plate to the one of the two principal surfaces of the recordingmedium, and coupling the conventional volume hologram to another surfaceof the glass plate using a material that includes gelatin.

The method may further include coupling a second prism to a back side ofthe volume hologram, and directing another radiation beam at the secondprism such that the other radiation beam passes first through the secondprism, the volume hologram, and the first prism to form an interferencepattern with portions of the radiation beam that reflected from theobject, wherein the radiation beam that reflected from the object andthe other radiation beam progress, at least partly, in a similardirection.

In yet a further aspect, a system to record a holographic image isdisclosed. The system includes a recording medium having two principalsurfaces and configured to record a holographic image of a volumehologram of an object placed proximate one of the two principal surfacesof the recording medium and a prism coupled at a contact surface of theprism to the other of the two principal surfaces of the recordingmedium. The system also includes a light source to direct light beams atanother surface of the prism such that such that a resultant radiationbeam reflected from the object interferes with a resultant firstreference beam progressing in a direction that is at least partlysimilar to the progression direction of the radiation beam reflectedfrom the object, both beams being separated by a small angle such that afirst interference fringe structure is formed in the recording mediumthat is inclined at an angle with respect to any of the two principalsurfaces of the recording medium, and such that a resultant secondreference beam progresses in a general direction that is at least partlyopposite to the direction of the radiation beams reflected from theobject such that a fine-sized set of interference fringes is formed inthe recording medium, the second set of interference fringe beinginclined with respect to the any of the two principal surfaces of therecording medium.

Embodiments of the system may include any one of the features describedin relation to the structure, methods and system above.

Details of one or more implementations are set forth in the accompanyingdrawings and in the description below. Further features, aspects, andadvantages will become apparent from the description, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial two-dimensional cross-section view of an embodimentof a resultant surface relief structure.

FIG. 2 is a schematic diagram illustrating a technique to form an Aztecdiffraction grating.

FIG. 3 is a partial cross-section view of an embodiment of a symmetricAztec grating formed using the technique illustrated in FIG. 2.

FIG. 4A is a schematic diagram of an exemplary system for recordingholographic images.

FIG. 4B is a flow diagram of an exemplary procedure to recordholographic images.

FIG. 5A is a schematic diagram of another exemplary embodiments of arecording system, with a prism, to record holographic images is shown.

FIG. 5B is a flow diagram of an exemplary procedure to recordholographic images.

FIG. 6A is a schematic diagram of apparatus for producing a volumehologram of a diffuser.

FIG. 6B is a schematic diagram of another configuration of the exemplaryembodiment of the apparatus of FIG. 6.

FIG. 7 is a schematic diagram of an exemplary embodiment of aholographic recording system coupled to a volume hologram.

FIG. 8A is a schematic diagram of another exemplary embodiment of ahologram recording system that uses S-Polarized incident light and dualprism coupling.

FIG. 8B is a schematic diagram of another exemplary embodiment of ahologram recording system that uses P-Polarized incident light for finefringe structure formation.

FIG. 9A is a groove profile of a ten step symmetric Aztec grating.

FIG. 9B is a graph of the diffraction efficiency versus the wavelengthfor the grating of 9A.

FIG. 10A is a groove profile of a ten step asymmetric Aztec grating.

FIG. 10B is a graph of the diffraction efficiency versus the wavelengthfor the grating of FIG. 10A.

FIG. 11A is a groove profile of a ten step skewed grating.

FIG. 11B is a graph of the diffraction efficiency versus the wavelengthfor the grating of 11A, with the grating having a period of 4 micronsand a step height of 250 nanometers.

FIG. 11C is a graph of the diffraction efficiency versus the wavelengthfor the grating of FIG. 11A, with the grating having a period of 4microns and a step height of 200 nanometers.

FIG. 11D is a graph of the diffraction efficiency versus the wavelengthfor the grating of FIG. 11A, with the grating having a period of 4microns and a step height of 200 nanometers and with the grating havingbeen overcoated with a clear dielectric having an index of refraction ofn=1.5.

FIG. 12A is a groove profile of a ten step skewed grating, where the topstep is shifted to a position 0.65 of the period.

FIG. 12B is a graph of the diffraction efficiency versus the wavelengthfor the grating of 12A, with the grating having a period of 4 micronsand a step height of 250 nanometers.

FIG. 13A is a photograph of a skewed blazed Aztec grating fabricated inphotoresist.

FIG. 13B is a groove profile of a ten step arbitrarily skewed grating,where the top step is shifted to a position 0.7 of the period.

FIG. 13C is a graph of the diffraction efficiency versus the wavelengthfor the grating of FIG. 13B, with the grating having a period of 4microns and a step height of 250 nanometers.

FIG. 13D is a graph of the diffraction efficiency versus the wavelengthfor the grating of FIG. 13B, with the grating having a period of 4microns and a step height of 350 nanometers.

FIG. 13E is a graph of the diffraction efficiency versus the wavelengthof the grating of FIG. 11B, with the grating having a period of 4microns and a step height of 200 nanometers and overcoated with a cleardielectric having an index of refraction of n=1.5.

FIG. 14 is a graph of the diffraction efficiency versus the wavelengthfor an asymmetric grating having a period of 4 microns and five steps,each with a step height of 250 nanometers.

FIG. 15 is a photograph of an embossed replica in plastic of a blazedAztec diffuse hologram.

FIG. 16 is a graph of the diffraction efficiency versus the wavelengthfor a four level crossed grating having a period of four microns in thex-direction and twelve microns in the y-direction and whereby the stepheight is 250 nanometers and the total depth is 1000 nanometers.

FIG. 17 is a graph of the diffraction efficiency versus the wavelengthfor a ten level crossed grating having a period of four microns in thex-direction and twenty microns in the y-direction, and in which the stepheight is 250 nanometers and the total depth is 2500 nanometers.

FIG. 18 is an isometric view of an asymmetric crossed four-step gratingin both x- and y-directions.

FIGS. 19A, C and E are exemplary grating profiles.

FIGS. 19B, D and F are graphs of the diffraction efficiency as afunction of grating depth for the profiles of FIGS. 19A, C and E,respectively.

FIGS. 20A-C are graphs of the diffraction efficiency as a function ofwavelength for the grating profiles of FIGS. 19A, C and E, respectively.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Described herein is a surface relief volume reflective diffractivestructure used, for example, to record holographic images. The structureincludes a recording medium, for example, a photoresist-based medium,that is configured to be structurally modified when exposed tointerfering and non-interfering portions of radiation beams. Thestructurally modified recording medium includes, when viewed in atwo-dimensional cross-section along one of the axes of the recordingmedium a plurality of equally spaced steps of fine-sized periodicitysuperimposed upon a plurality of deep depressions of substantiallycoarse-sized periodicity (i.e., the period of the plurality of the deepdepressions is larger as compared to the fine-sized periodicity of thesteps in the depressions). The structurally modified recording medium isconfigured to produce single and multiple colors in a broad spectralrange in reflection when illuminated by a source of light. That is, theresultant modified structure of the recording medium is such that whenthe structure is illuminated with a source of light, reflected lightcorresponding to a particular point of the reproduced image will appearas substantially a single color (that color being one from a widespectrum of available colors that can be reproduced). In contrast,conventional surface relief structures produce images that include, atleast partially, a rainbow of colors (i.e., a prism-like effect).Furthermore, conventional volume holograms produce images with colorsdrawn from a relatively narrow range of the color spectrum.

Referring to FIG. 1, a partial two-dimensional cross-section view 100 ofan embodiment of a resultant surface relief structure configured toproduce single and multiple colors in a broad spectral range inreflection is shown. When viewed in cross-section, the surface reliefstructure includes a sequence of periodic deep depressions, such as deepdepression 102, upon which equally spaced steps 104 a-e, having a period106, are superimposed. The steps are equally spaced in that they are ofare of substantially equal height. As will become apparent below, theparticular height of steps is related to the resonant wavelength thatwill be reflected from the formed structure when the structure isilluminated (e.g., the resonant wavelength is twice the step heighttimes the index of refraction of the overcoating dielectric layer). Insome embodiments, and as shown in FIG. 1, the deep depressions 102 areskewed relative to the edges of the two-dimensional cross-section viewof the recording medium, and are thus skewed relative to the principalsurfaces of the modifiable recording prior to the exposure of therecording medium to radiation beams (i.e., before the recording mediumwas etched), in that the orientation of the formed depressions isinclined. The deep depressions and steps superimposed thereon aregratings that cause reflected illumination to produce an imagecorresponding to the formed grating structure. As will be described ingreater detail below, grating profiles that are skewed (e.g., inclinedand/or non-symmetric) have a diffraction efficiency (i.e., spectralresponse) that is more concentrated near the resonant wavelength, thusresulting in apparent single color reflections (corresponding to aresonant reflection plus contributions from near-by side lobescorresponding to neighboring wavelength) when the structure isilluminated.

The deep depressions are also referred to off-axis opening fringestructures in that the openings (i.e., depressions) are inclined atangle relative to the normal axis of the cross-section view. As shown inFIG. 1, the sequence of deep-depressions has a period 108. As shown, theperiod 108 of the depressions 102 is generally larger than the period106 of the steps 104 and thus, under these circumstances, the period 108is referred to as having a coarse-sized periodicity in comparison to thefine-sized periodicity of the steps 104. In some embodiments the periodof the deep depressions is ten (10) times, or more, larger than theperiodicity of the steps (e.g., the depressions could have a period of3.5-4 μm compared to the 200-400 nanometer periodicity of the steps).The period 108 of the deep depressions corresponds to off-perpendicularfringes that would be obtained in conventional surface reliefstructures. Projecting these off-perpendicular fringes onto the surfacesof the cross-section view 100 of the recording medium results inprojections 110. As will further be discussed below, the procedures andsystems for recording holographic images provide great leeway andflexibility in terms of the resultant surface structure. Resultantsurface relief structures can thus have profiles ranging from nearlysymmetrical configurations to asymmetric configurations. The variationbetween symmetric and asymmetric configurations determines, at least inpart, the resultant spectrum of the diffracted light. Asymmetricstructures cause a more defined color spectrum to be generated inreflection of the structure such that respective points of the reflectedimage will generally correspond to specific single color componentsrather than to a continuous rainbow of colors. Furthermore, skewing thegrating of the structures formed in the recording medium also enablescontrolling the color spectrum of reflected light such that reflectionsfrom the gratings of the recording medium (or a replica thereof) aremore defined and include apparent single color components from a widespectrum of possible colors. As described herein, skewing of the gratingrefers to causing the resultant grating structure to be inclined withrespect to the principal surfaces of the recording medium and/or causingthe grating structure to have a non-symmetric configuration (i.e., aconfiguration that is not entirely symmetric, yet not entirelyasymmetric, and in which the center point of the stepped pyramid, theapex, is displaced somewhat so that it does not occur exactly at thehalf-way point of the period of the grating profile).

In contrast to the skewed coarse-sized grating structure shown in FIG.1, FIGS. 2 and 3 show a two-dimensional cross-section view of anon-skewed grating profile 120. FIGS. 2 and 3 also schematicallyillustrate the recording configuration used to form the interferencefringes in the recording medium. Specifically, off-axis beams (e.g.,laser beams) 122 and 124 interfere to produces the sets of interferencefringes 130 a-f which are predominantly perpendicular (normal) to theprincipal surfaces of the recording medium, such as a photoresistmedium, prior to exposure of the recording medium to the radiation beamsthat modify the structure to form fringe patterns in the medium. Boththe radiation beams 122 and 124 are incident from the same side of themedium onto the top edge 134 of the cross-section view 120 of therecording medium. These two beams thus enable the formation of the deepdepressions in the recording medium corresponding to the interferencefringe pattern. Two other radiation beams (e.g., also laser beams) 126and 128, are incident on the recording medium from opposite sides of theprincipal surfaces of the recording medium, and incident at aperpendicular orientation to the surfaces, thus resulting in theinterference fringes 132 a-d which are substantially parallel to theprincipal surfaces of the recording medium (prior to its exposure to theradiation beams that thereafter cause modification to the structure ofthe recording medium).

With reference to FIG. 3, showing the etched cross-section view 120, theinterfering patterns resulting from the off-axis radiation beams 122 and124 include deep depressions having a general sine wave profile 136. Asshown, the general sine wave profile 136 is modulated into a steppedprofile, with the steps corresponding to the interference planes of thecounter propagating radiation beams 126 and 128. The deep depressionsformed by the off-axis radiation beams 122 and 124 enable the lowerlying fringes (e.g., fringe 132 d, to be accessed. Consequently, theresultant modified structure of the recording medium which includes thestepped deep depressions can be mechanically replicated. However, asnoted above, when the recording medium modified so that it hasnon-skewed deep depression modulated by step is illuminated by light,the reflected light will generally be confined to a relatively narrowsubset of the color spectrum. Additionally, the non-skewed gratingpattern may give rise to multiple color reflections at some off-axisviewing angles.

Referring to FIG. 4A, a schematic diagram of a system 140 for recordingholographic images by producing skewed fringe patterns in a surfacerelief structure such that the resultant structure can achieve singlecolor reflections upon illumination of light on the recorded image isshown. The system 140 includes at least one radiation source 142 toproduce radiation beams that are directed at the recording medium 144(shown in cross-section for ease of explanation) that configured to bestructurally modified when exposed to interfering and non-interferingportions of radiation beams. The recording medium 144 includes, in someembodiments, two principal surfaces, 145 a and 145 b. Radiation directedfrom the radiation source 142 is typically incident on one of theprincipal surfaces of the recording medium (the surface 145 s in theillustration of FIG. 4). In some embodiments, the radiation beamsinclude, for example, at least one of light beams, produced by a lightsource such as a laser source, and/or a source to produce electronbeams. The nature and composition of the recording medium will depend onthe radiation source used. For example, in circumstances in which lightradiation is used to record holographic images, as described herein, therecording medium used is, for example, a photoresist recording medium(e.g., a positive photoresist that is etched when exposed to radiation).In some embodiments, a recording medium that responds to particle (e.g.,electron) beam radiation may be used in circumstances where particlebeams radiation is used by the recording system.

An object 146 whose image is to be holographically recorded is disposedproximate to another of the principal surfaces of the recording medium144 (in this case, the principal surface 145 b). In some embodiments,the object 146 may be encapsulated in a clear, index-matching materiallike optical cement 148 defining a contact surface 149 that interfaceswith the principal surface 145 b of the recording medium 144. Inoperation, the at least one radiation source 140 produces radiationbeams 150 directed at the recording medium 144 from a direction thatincident to the principal surface 145 a. In some embodiments, theradiation beams are directed at an off-axis angle relative to the normalto the principal surface 145 a of the recording medium 144 (e.g., anangle of more than 10° from the normal to the principal surface 145 a).

The produced radiation beams 150 are incident, at the particular angle,on the recording medium 144. At least one portion of the radiation beams150, namely the portion 152, passes through the recording medium 144 andthrough the encapsulate 148 and is reflected from the object 146 asradiation beam 154. At the same time, another portion 156 of theradiation beams 150 if reflected at the encapsulate-photoresistinterface (i.e., at the point where the principal surface 145 b and thecontact surface 149 of the encapsulate 148 meet) as beam 158. Generally,the path taken by the beams (i.e., whether one portion will pass throughthe recording medium while another portion will reflect at the recordingmedium) depends on several factors, including the angle of incidence,the reflectance of the object, the polarization of the incident beam,the index of refraction of the cover glass and the underlyingphotoresist layer. For example, for light polarized parallel to theinterface, the reflectance at the interface increases with angle ofincidence, while for the opposite polarization, there is an angle,namely, the polarization angle, where the reflectance drops to zero.Thus, the ratio of light that is directed to the object or reflected atthe recording medium can be varied, for example, by changing the stateof polarization. Interference between the reflected radiation beams 150and 154 (i.e., the beams incident from opposite sides of the recordingmedium 140) produce the so-called step fringes (e.g., similar to thestep fringes 104 a-e in FIG. 1) whereas the interference between thebeams 154 and 158, which progress at least partly in a similardirection, produces the opening fringes (i.e., the deep depression suchas depression 102 in FIG. 1). Upon exposure and development, a deep,stepped non-symmetric profile results. Thus, the system 140 isconfigured to direct radiation beams that produce off-axis referencebeams that combine with a reflected object beam in such a way that atleast one off-axis reference beam (e.g., beam 158) progresses, at leastpartly, in a similar direction as the object beam (e.g., the x and/or ydirection orientation of the vector representation of the reference andobject beams is the same) and such that at least one reference beam(e.g., beam 152) progresses in at least partly an opposite direction tothe reflected object beam 154 (e.g., the x and/or y directionorientations of the vector representations of the reference and objectbeams are substantially opposite to each other). The interference of thevarious beams produces skewed (i.e., inclined and/or non-symmetric)interference fringe patterns (e.g., such as those shown in FIG. 1) thatcan produce substantially single color components from a broad spectralrange in reflection when illuminated by a source of light. For example,if the incident light had wavelength 457 nm and was incident at a 60degrees angle to the normal of the surface of the recording medium, thenthe angle of incidence of the radiation beam at the object would be 10.6degrees (based on Snell's law and assuming a tilt angle of 10 degreesfor the object, and an index of refraction for photoresist of n=1.7).Under these circumstances, the fringe spacing isd=457/(2×1.7×cos))(20.7°))=144 nm. If the recorded hologram on therecording medium were replicated and embossed into plastic having anindex of refraction of n=1.5, and the hologram image was subsequentlyviewed through the plastic base, the observed color would havewavelength=432 nm, which is a blue-violet color. To access the rest ofthe visible spectrum a different recording geometry would have to beused.

Referring to FIG. 4B, a flow diagram of an exemplary procedure 180 torecord holographic images is shown. Initially, a recording medium, suchas, for example, the recording medium 144, is provided 182. Therecording medium provided has, in some embodiments, two principalsurfaces (e.g., 145 a and 145 b in the case of the recording medium 144)and is configured to record a holographic image. An object is placed 184proximate to one of the two principal surfaces of the recording medium.For example, the object, whose image is to be holographically recordedis placed proximate to the surface 145 b of the recording medium.Radiation beams (e.g., light beams) are then directed 186 at the othersurface of the recording medium (i.e., the other surface that is notimmediately next to the object whose image is to be recorded). Theradiation beams can be directed from a light source positioned toilluminate radiation on the recording medium. The radiation beams aredirected at an angle with respect to a normal to the other of the twoprincipal surfaces of the recording medium such that a resultantradiation beam reflected from the object (e.g., like radiation beam 154in FIG. 4A) interferes with a resultant first reference beam progressingin a direction that is at least partly similar to the progressiondirection of the radiation beam reflected from the object. The twointerfering beams may be separated by a small angle such that a firstinterference fringe structure is formed in the recording medium that,for example, is inclined at an angle with respect to any of the twoprincipal surfaces of the recording medium. Additionally, directing theradiation beams at the recording medium also results in a secondreference beam that progresses in a general direction that is at leastpartly opposite to the direction of the radiation beams reflected fromthe object such that a fine-sized set of interference fringes is formedin the recording medium. That second set of interference fringe may alsobe, for example, inclined with respect to the any of the two principalsurfaces of the recording medium. The resultant interference fringepatterns may also be non-symmetric.

Referring to FIG. 5A, a schematic diagram of an exemplary embodiments ofan alternative recording system 160 to record holographic images isshown. The system 160 includes at least one radiation source 162. Theradiation source 162 may be, for example, a light source illuminatinglight. The system 160 includes a recording medium 164 (which may besimilar to the recording medium 144) and an object 166 that is disposedproximate to one of the principal surfaces of the recording medium 144(namely, principal surface 165 b). In some embodiments, the object 166may be encapsulated in a clear, index-matching material like opticalcement 168 defining a contact surface 169 that interfaces with therecording medium 164. As shown, the system 160 further includes anoptical coupling device, such as a prism 161, that couples theilluminated source radiation onto the recording medium and the object(or encapsulated object). In some embodiments, a suitable opticalcoupling device is an equilateral prism having an index of refraction ofn=1.5. The prism 161 is coupled to the recording medium using, forexample, a closely index matching liquid. Examples of suitable couplingliquids that may be used include, for example, xylene (having an indexof refraction n=1.49), decalin (n=1.48) and/or water (n=1.33).

As will be described below in greater detail, in some embodiments, therecording systems and procedures described herein can be used to recordconventional volume hologram into a surface relief structure. Underthose circumstances, to properly couple the recording medium 164 onwhich the prism 161 is disposed to such a volume hologram, the recordingmedium can be coupled to a separate thin glass plate (e.g., coatingphotoresist onto the separate thin glass plate) and the glass plate (notshown) can be coupled to the volume hologram using, for example, gelatin(e.g., a mixture that includes one part gelatin powder or crystal mixedwith four parts water which is then allowed to swell, and issubsequently melted down at a temperature of around 60 degrees C.). Whenthe gel is applied in the molten state, it will solidify at roomtemperature. After exposure of the recording medium the gelatin mixturelayer can simply be stripped away, allowing the photoresist to bedeveloped.

With continued reference to FIG. 5A, radiation beams 170 produced by theradiation source are directed at the prism 161 and are thus incident ata prism side at an incident angle of, for example, 60° relative to thenormal to the principal surface 165 a of the recording medium. The prism161 causes a portion of the radiation 172 to be directed so that itpasses the recording medium 164 and is reflected off the object 166.Another portion of the radiation beam 170, namely, portion 176, isreflected at the recording medium to result in a reflected referencebeam 178. The ratio of the various radiation portions resulting fromdirecting the radiation beam at the prism 161 will depend, at least inpart, on the polarization state of the incident beam. Thus, theconfiguration of the system 160 shown in FIG. 5 results in an objectradiation beam reflected from the object 166, a reference beam 176 thatprogresses, at least partly, in a direction substantially opposite thedirection of progression of the object radiation beam 174 (e.g., the xand/or y direction orientations of the vector representations of thereference and object beams are substantially opposite to each other),and a second reference beam 178 progression in a direction that is atleast partly similar (within a small angle separation) to theprogression direction of the object radiation beam 174 (e.g., the xand/or y direction orientation of the vector representation of thereference and object beams is the same). The reflected object beam 174interferes with the reflected reference beam 178 to form a deepdepression fringe pattern (similar to the fringe pattern that includesthe deep depression 102 of FIG. 1). That pattern is generally skewed(non-symmetric and/or inclined) relative to the principal surfaces ofthe recording medium (prior to exposure of the recording medium toradiation beams). The reflected object radiation beam 174 alsointerferes with the reference radiation beam 176 to form skewed stepfringes (similar to the steps 104 a-e shown in FIG. 1).

As an illustrating example of the fringe structure that results fromrecording a holographic image using a prism as described in relation toFIG. 5, source radiation is directed at a prism coupled to a photoresistrecording medium in a manner similar to that shown in FIG. 5. The angleof incidence of the source radiation is 60 degrees and the object istilted at 10 degrees (similar to the setup for the example used inrelation to FIG. 4). When a prism is used in the recording procedure, weobtain fringe spacing of d=457/(2×1.7×cos 50°)=209 nm. The observablewavelength at normal incidence through an embossed plastic base wouldthen be (209×2×1.5)=627 nm, corresponding to a red color. Thus, use ofprism coupling therefore enables access to a more complete set of thevisible spectrum than was possible using, for example, the system 140shown in FIG. 4.

Referring to FIG. 5B, a flow diagram of an exemplary procedure 190 torecord holographic images is shown. Initially, a recording medium, suchas, for example, the recording medium 164, is provided 192. Therecording medium provided has two principal surfaces (e.g., 165 a and165 b in the case of the recording medium 164) and is configured torecord a holographic image. An object is placed 194 proximate to one ofthe two principal surfaces of the recording medium. For example, theobject, whose image is to be holographically recorded is placedproximate to the surface 165 b of the recording medium. A prism (e.g.,an equilateral prism) is coupled 196 at the contact surface of the prismto the other principal surface of the recording medium. Radiation beams(e.g., light beams) are then directed 198 at another surface of theprism (i.e., a surface not coupled to the recording medium). Theradiation beams can be directed from a light source positioned toilluminate radiation on the recording medium. The radiation beams aredirected such that a resultant radiation beam reflected from the object(e.g., like radiation beam 174 in FIG. 5A) interferes with a resultantfirst reference beam progressing in a direction that is at least partlysimilar to the progression direction of the radiation beam reflectedfrom the object. The two interfering beams may be separated by a smallangle such that a first interference fringe structure is formed in therecording medium that is inclined at an angle with respect to any of thetwo principal surfaces of the recording medium that interfering patternmay also be non-symmetric. Additionally, directing the radiation beamsat the recording medium also results in a second reference beam thatprogresses in a general direction that is at least partly opposite tothe direction of the radiation beams reflected from the object such thata fine-sized set of interference fringes is formed in the recordingmedium. That second set of interference fringe may also be inclined withrespect to the any of the two principal surfaces of the recordingmedium.

In some embodiments, recording of a holographic image is performed byplacing a conventional volume hologram of an object proximate to one ofthe principal surfaces of the recording medium instead of placing theactual object proximate the recording medium. In other words, in suchembodiments, the procedure for recording holographic images of an objectincludes an initial recording of a conventional volume hologram of theobject, and then using the recorded volume hologram to produce a surfacerelief structure using a system such as, for example, system 140 or 160,and performing the operations required to produce holographic imagesusing those system configurations. Referring to FIG. 6A, a typicalholographic apparatus 200 for producing a volume hologram of a diffuseris shown. A radiation beam, such as laser beam 202, is incident onexpansion lens 204. The resultant expanded beam 205 impinges on adiffuser plate 206. In some embodiments, the diffuser plate 206 couldbe, for example, a ground glass plate. A diffuse beam 208 expandsfurther until it is incident on the recording volume hologram plate 210.The hologram material could be, for example, dichromated gelatin (DCG),photopolymer and/or silver halide.

An exposed and developed hologram recording, for example, a recording ofan object performed on a plate of DCG can subsequently be used to recorda holographic image of the object on a surface relief structure (whichcan subsequently be mechanically replicated) instead of using the actualobject to record the holographic image. Additionally and/oralternatively, the resultant conventional volume hologram can be furtherprocessed. Referring to FIG. 6B, a schematic diagram of an exemplaryembodiment of another mode of operation for the apparatus 200 of FIG. 6Ais shown. Particularly, the radiation paths defined by the radiationbeams 212 (in this case, the radiation beams are light beams) aredirected at the volume hologram plate 210. The incident light beams thuscause a second focused image of the hologram image recorded on thevolume hologram to form on the diffuser plate 206 (the focused image isrepresented as the dotted image on the diffuser plate 206). Incircumstances in which the volume hologram plate is based on DCGmaterial, blue light recorded on the DCG plate can be swelled chemically(e.g., subjecting the plate to a series of water and alcohol baths atdifferent temperatures and by immersion is a chemical calledtriethanolamine (TEA), such that the fringe spacing is enlarged to thusenable reconstruction of the image with red or green light instead ofblue.

Once a conventional volume hologram has been prepared, the processedvolume hologram plate 210 with a holographic recorded image of an objectcan thus be coupled to the prism in a manner similar to that performedin relation to the coupling of the encapsulated object 166 to the prism161 in FIG. 5. Referring to FIG. 7, a schematic diagram of an exemplaryembodiment of a holographic recording system 220 is shown. As in thesystem 160 shown in FIG. 5, the system 220 includes a radiation source,such as light source 222, that directs light incident, at an angle, at aside of a prism 221 (e.g., an equilateral prism). The prism 221 iscoupled to a recording medium 224, such as a photoresist medium,configured to be structurally modified when exposed to interferingand/or non-interfering radiation beams. The light source 220, the prism221 and the recording medium 224 may be similar to the light source 160,the prism 161 and the recording medium 164 described in relation to thesystem 160 of FIG. 5. Coupled to a principal surface 225 a of therecording medium 224 is the volume hologram plate 210 describe inrelation to FIGS. 6A and 6B. Recorded on the hologram plate 210 is aholographic image 213 corresponding to some tangible object previouslyholographically recorded on the plate 210. As shown, the recorded imagewas recorded with a built-in tilt angle.

An advantage of using a volume hologram to record holographic images ona surface relief structure instead of using the actual object is thatthe recorded holographic image of the volume hologram can be focused tobe flat in an image plane and yet still be reconstructed at an anglewithout distorting the image. That is, to an observer observing, forexample, a holographic image at a common viewing angle, e.g., 20degrees, the image will appear flat and undistorted even though theimage is being viewed at an angle. Moreover, because of the transparencyof narrow-band volume holograms to off-axis light, the use of a volumehologram to record holographic images on a surface relief structureprovides more flexibility in terms of the amount of light that isdistributed between the off-axis exposure and the volume step structureexposure. Specifically, if the volume hologram is recorded, developed,and processed such that it is nearly monochromatic with a narrow bandsingle color, then the reconstruction efficiency becomes sensitive tovariations in the angle of incidence of the reference beam onto thehologram. If the angle is exactly at resonance, then the hologram willreconstruct with high efficiency. A slight angular variation off theresonance will generally result in almost no reconstructed light, andthe reference beam will almost entirely be transmitted through thehologram plate. Because with prism coupling the angle of incidence isbeyond the critical angle for total reflection (given as sin(θ)=1/n,where n=index of refraction; if n of the prism=1.5, the critical angleof total reflection is θ=41.8 degrees), this beam will be totallyreflected at the prism-air interface. Accordingly, through carefuladjustment of this angle, the amount of light totally reflected can bechanged from nearly zero to some non-zero value. For the on-resonancecondition, most of the reflected light is from the object and thus thestep structure fringe formation is maximized through the interference ofbeams 230 and 234. If the angle is slightly off-resonance, then morelight is totally reflected as beam 238 interferes with the object light234 to form the off-axis fringes. Exposure can be varied between thesetwo conditions to maximize the resultant effect on the eventual etchedgroove profile in photoresist.

The production of more than one color can be done by recordingmulticolor volume holograms. One technique that can be used to implementthe recording of multicolor volume holograms is to stack togetherseparate plates corresponding to different colors.

Referring to FIG. 8A, a schematic diagram of another exemplaryembodiment of a hologram recording system 250 is shown. The system 250is based on a two prism and a split beam configuration. Theconfiguration of the system 250 takes into account the polarization ofthe beams. A volume hologram plate 260 (which may be similar to theplate 210 described in relation to FIGS. 6 and 7) is positioned so thatit is coupled to a recording medium 254 such as a photoresist mediumconfigured to be structurally modified when exposed to interferingand/or non-interfering radiation beams at a surface 261 of the plate260. The recording medium 254 is coupled to a prism 251 which may besimilar to the prisms previously described in relation to FIGS. 5 and 7.The system 250 includes an additional second prism 252, which may alsobe an equilateral prism, that is coupled to the back side of thehologram plate 260, namely, at the surface 262 of the plate 260. Theincident beam 270 generated by the radiation source 252 is split suchthat one portion of the split beam is incident on the hologram plate toilluminates the recorded holographic image 263 recorded on the plate260. The second split beam 272 is brought into the second prism 252 froma beam splitter (not shown) between the incident laser beam and the twoprisms. Generally, one of the beams from the beam splitter proceeds tothe first prism while the other beam from the beam splitter goes to thesecond prism. The second split beam 272 passes through the prism 252 atan off-resonant angle to interfere with reflected beam 274 to form theoff-axis fringes. This is done with s-polarized light (i.e., E-vectorlying in a plane normal to the optical table, as shown). This light isused substantially entirely to form the off-axis fringes, because beams270 and 272, as well as beams 270 and 274 interfere only minimally as aresult of their E-vectors being approximately orthogonal. With referenceto FIG. 8B, if the split beam can be adjusted so that all the light goesinto beam 270 at the resonant angle and the polarization is changed top-polarization (E-vector parallel to the photoresist interface), thenformation of the step fringes is maximized by interference between beams270 and 274.

It should be noted that in the above recording systems and methods, thephotoresist should be in close contact with either the actual object ora hologram of the object. This is because the fringe structure is sofine (of submicron dimensions), that any movement has a deleteriouseffect on the fidelity of the fringes. Typical volume holograms avoidthis because they include multiple layers. Even though each layer maydiffract with a small efficiency, the overall effect is to produce abright result through the additive effect of the many layers. With Aztecstructures, on the other hand, there is really only one stepped layer,so its fidelity has to be good to achieve high efficiency.

To investigate the performance of surface relief structures such asthose described herein, a computer application program GSolver™,provided by the Grating Solver Corporation, was used to compute thediffraction efficiency achieved with several surface relief gratingstructures. The GSolver™ enables special geometries to be considered,such as partial, skewed (including inclined and/or non-symmetricgeometries), symmetric and asymmetric profiles. The application computesthe diffraction efficiency corresponding to those geometries. For thepurpose of the analysis provided herein, several gratingconfiguration/geometries were considered, including fully symmetricaland fully asymmetrical grating geometries. The gratings profilesconsidered corresponded to surface relief structures having step heightsand dielectric overcoatings that provide viewability in the visiblespectral range. Such grating profiles may be produced using therecording systems described, for example, in relation to FIGS. 4, 5, 7and 8.

Referring to FIGS. 9A and 9B, a partial cross-section view of asymmetric grating profile 300 and a diffraction efficiency graph 310 arerespectively shown. The grating profile 300 is a simplified ten levelsymmetrical grating structure. FIG. 9B shows the results of the fullvector diffraction efficiency computations as a function of wavelengthfrom 300 to 800 nanometers, using a grating period of four micrometers(4 μm) and a step height of 250 nanometers. For the purpose of theanalysis and computation, the steps of the structure were assumed to besteps coated with an opaque aluminum layer, and the assumed overcoatingdielectric used was air having an index of refraction of n=1.0. As shownin FIG. 9B, resonance for the grating profile 300 occurs at a wavelength500 nanometers, which corresponds to twice the step height. In addition,as can be seen, there are multiple subsidiary spectra (side lobes) atwavelengths adjacent to the resonant wavelength, but at substantiallylower efficiencies.

Referring to FIG. 10A, an asymmetric ten level grating profile 320 isshown. As with the grating profile 300 shown in 9A, the grating profile320 has steps having a height of 250 nanometers and are coated with anopaque aluminum layer. The overcoating dielectric used was again airhaving an index of refraction of n=1.0. FIG. 10B shows the results ofthe diffraction efficiency computation as a function of the wavelength(for a wavelength range of 300 to 800 nanometers). The use of steps witha height of 250 nanometers once again results in a resonance wavelengthof 500 nanometers for the grating profile 320. On the other hand, asshown in the graph of FIG. 10B, the subsidiary spectra, which previouslyhad low efficiency values (indicating that other wavelength are greatlyattenuated or suppressed for the profile 300) now include wavelengthscorresponding to high diffraction efficiency, thus indicating that forthe grating profile 320, multiple colors would be viewed in reflectionwhen that structure is illuminated. In fact, the whole spectral rangecould fit under an envelope that is typical of a shallow blazed sawtoothgrating.

Referring to FIG. 11A, a surface relief structure profile 340 for a tenlevel, nearly asymmetric Aztec grating is shown. In the Grating Solverprogram, this profile is specified as a skewed non-symmetric profile,where the center point at the top is displaced from the 0.5 center pointof the period to 0.75 of the period. As with the grating profiles 300and 320 of FIGS. 9A and 10A, the grating profile 340 has step height of250 nanometers. As described herein, skewed gratings, in which thestepped and deep depression fringes formed in the surface reliefstructure are skewed relative to principal surfaces of the recordingmedium, can be produced using the systems and procedures described, forexample, in relation to FIGS. 4, 5, 7 and 8. As also described herein,skewing the grating profile can also be achieved by causing the profileto be inclined at an angle relative to the principal surfaces of therecording medium. FIG. 11B shows the spectral responses, expressed asthe diffraction efficiency for a wavelengths range of 300-800nanometers. As shown, resonance for the grating profile 340 once againoccurs at 500 nanometers. However, unlike the spectral response for thegrating profile 320, as shown in FIG. 10B, in this case the diffractionefficiency of spectra adjacent to the resonant wavelength is attenuatedand bunched closer to the resonance. Thus, the spectral response for thestructure 340, having the skewed grating profile, is such that it has anapparent single color at the 500 nanometers resonance wavelength.Although the spectral response shown in FIG. 11B is not one thatcorresponds to a pure narrow-band color, the side lobes adjacent to theresonant wavelength are substantially centered at the 500 nanometerwavelength that any rainbow effect when the reflection from a physicalstructure having a grating profile similar to that of the gratingprofile 340 would be reduced as compared, for example, to the gratingprofile 320 of FIG. 10A. As also seen, the side lobes of the resonantwavelength corresponding to the −1 to −4 spectra (i.e., the lobescorresponding to wavelength from 350 nanometers to around 470 nanometer)have higher spectral contributions (i.e., higher diffraction efficiency)than the side lobes corresponding to the wavelengths 530-730 nanometer.These higher contributions from the negative side lobes, along with thecontribution from zero order lobe (i.e., the resonant wavelength)constitute a blue and green combination, which means that the apparentcolor observed in reflection is closer to cyan color than to green.Because more colors are taken out of the spectrum with respect to asingle narrow-band color, the brightness would be significantly greater.Furthermore, the focus of the resultant image would still be retainedbecause that focus depends on the narrow band behavior of each singlewavelength in the cluster. Furthermore, if the spectral scan refers toan actual holographic image, the negative side lobes in the spectralresponse, along with the zero-order, are all part of the real image,while the positive orders would correspond to the pseudoscopic image(i.e., an image turned inside out).

FIG. 11C shows the results of the diffraction efficiency computationsfor the grating profile shown in FIG. 11B when the step height for thegrating profile 340 has been changed from 250 nanometer to 200nanometers. Because the resonance of the stepped pyramid structuresdepends on the step height (for a dielectric with an index of refractionof n=1, it is twice the step height), as expected, the resonance in thegraph of FIG. 11C appears at 400 nanometers. As further shown in thegraph of FIG. 11C, in addition to the resonance lobe there are multipleadjacent side lobes that contribute to the overall brightness in amanner similar to that described in relation to FIG. 11B. Thus, for themodified structure corresponding to graph of FIG. 11C, an overallspectral shift from the green part of the spectrum (at 500 nanometers)to the blue part of the spectrum at approximately 400 nanometers isevident. FIG. 11D shows the results of the diffraction efficiencycomputations for a grating profile that has 200 nanometer steps coatedwith a clear dielectric having an index of refraction of n=1.5. Thistype of a grating profile could be obtained, for example, by embossing asurface relief structure with 200 nanometer step height pattern (formed,for example, through one or more of the procedures described herein inrelation to FIGS. 4, 5, 7 And/or 8) into a plastic base having an indexof n=1.5. As shown in FIG. 11D, the diffraction efficiency spectralresponse is shifted to approximately the theoretical resonant wavelengthof 600 nm (computed as λ_(resonant)=2 nh, which in this case is equal to2×200 nm×1.5=600 nm). Similar to the spectral response shown in FIGS.11B-C, the central resonance lobe is accompanied by several closelybunched side lobes having slightly less intensity, but whichnevertheless contribute to the overall color effect to produce anapparent single color. In this particular case the spectral responseincludes a resonant wavelength located at the red end of the visualspectrum with contributions from the green part of the spectrum. Thus,the apparent color produced is closer to orange color than to red.

Referring to FIG. 12A, a grating profile 360 having a slightly lessskewed profile than the surface relief structure of FIG. 11A is shown.The surface relief structure profile has a step height of 250 nanometersand the dielectric interface for the surface relief structure is air(i.e., n=1). The center point of the grating profile 360 is displacedfrom being at 0.5 of the period to 0.65 of the period. FIG. 12B showsthe results of the diffraction efficiency computations for the surfacerelief grating profile of FIG. 12A. As shown, the spectral response forthe grating profile 360 is similar to the spectral response for the forthe grating profile 300 of FIG. 9A (i.e., the total symmetric gratingprofile) except that that the spectral response for grating profile 360exhibits an increase in the diffraction efficiency of the side lobesadjacent to the center resonance of 500 nanometers. These adjacent lobes(corresponding to particular colors) cause a much brighter and apparentsingle color, centered at 500 nanometers (although less pure in terms ofcolor composition in comparison to, for example, symmetric gratingprofiles). The combination of colors from green to blue means that theapparent single color is closer to cyan than to green.

Referring to FIG. 13A, a photograph of an exemplary skewed non-symmetricblazed Aztec grating, fabricated in photoresist, is shown. FIG. 13Bshows a skewed profile 380 prepared such that it closely simulates theactual photoresist grating profiles obtained experimentally. The gratingprofile 380 is skewed in such a way that its center point is at 0.65 ofthe period, but the short side of the profile is skewed inward ascompared to the profile shown in FIG. 12A. FIG. 13C shows the results ofthe diffraction efficiency computations for the grating profile 380 (thestep height of the steps is 250 nanometers and the dielectric is airwith an index of refraction of n=1, thus resulting in a theoreticalexpected resonance wavelength of 500 nanometer). As shown, the resonancewavelength does indeed occur at approximately 500 nanometers. Thespectral response includes several adjacent side lobes which are morepredominant that the side lobes shown in FIG. 12. Accordingly, theresultant apparent color will likewise be approximately a cyan colorthat is more salient here than that shown in FIG. 12. FIG. 13D shows thediffraction efficiency computation results for the grating profile 380when the step height increased to 350 nanometers (an air interface isused here too). As shown, the central resonance wavelength occurs at 700nanometers (the expected value of two times the step height) and heretoo the central resonant component is accompanied by several adjacentside lobes that contribute to the overall brightness of the perceivedcolor reflected upon illumination of a surface relief structure withthis profile. The predominant color contribution in this case is red,with smaller contributions from a green color, thus yielding an apparentreddish-orange color. Referring to FIG. 13E, the diffraction efficiencycomputation results for the grating profile 380 in which the step heightis changed to 300 nanometers, and with steps coated with a cleardielectric having an index of refraction of n=1.5, is shown. Thespectral response shows resonances in both the red and blue ends of thespectrum, but little contribution from the lobes at around the greencolor. Thus the resultant apparent color is blue-red, or magenta.

FIG. 14 shows the diffraction efficiency computation results (i.e.,spectral distribution) for a totally asymmetric Aztec stepped profilehaving a period of three micrometers, and five steps at a spacing of 250nanometers each. This grating profile is not skewed (i.e., in that it istotally asymmetric). As shown in the graph of FIG. 14, this particularstructure results in a spectral response that includes three resonanceswithin the visible spectrum, at blue, green, and red, for the −1, 0, and+1 orders, respectively (i.e., the central resonance and its two sidelobes on either side of the central resonance). A hologram made in thisway would show, for example, a real orthoscopic image in the 0 and +1orders, and a virtual, pseudoscopic image corresponding to the −1 orderresonance. The pseudoscopic image would appear turned inside out withrespect to the real image, and it would appear on the opposite of thenormal to the real image. The zero order in this case corresponds to thereal image that would be observed with a standard volume hologram usingmaterials such as DCG or silver halide.

Referring to FIG. 15, a photo of an embossed replica 400 in plastic of ablazed Aztec hologram is shown. The object whose holographic image wasto be recorded was in this case a volume hologram of a diffuse surface,and was recorded as a volume hologram using, for example, a systemsimilar to the system 200 shown in FIG. 6. The holographic recording ofthe volume hologram on a surface relief structure was made in aphotoresist recording medium on glass. The glass was overcoated withvacuum evaporated silver and a nickel replica was made from it byelectroplating. The nickel replica was then used to emboss the holograminto plastic, which in this case was rigid vinyl, using a heat press.After two minutes in the heat press, the plastic replica was allowed tocool, and was then overcoated with a vacuum evaporated layer ofaluminum. The somewhat random nature of the surface is a reflection ofthe diffuse character of the object. The clearly defined step structureallows the diffuse image to be observed in an essentially single color.

FIG. 16 shows the diffraction efficiency computation results, as afunction of wavelength, for a four-level crossed asymmetric grating,with a step-size of 250 nanometers, having an x-direction period of 4000nanometers and a y-direction of 12000 nanometers. Such variations could,for example, be brought about by the introduction of certain variationsin the y-direction due to the diffuse character of the image. As shownin FIG. 16, the grating configuration resulted in large diffractionefficiency at the central zero-order resonance and attenuateddiffraction efficiency for the adjacent orders. FIG. 17 shows thediffraction efficiency computation results, as a function of thewavelength, for a ten-level crossed asymmetric grating, with a step sizeof 250 nanometers, in which the x-direction period is 4000 nanometersand the y-direction period is 20000 nanometers. As shown, the spectraadjacent to the central zero-order resonance (at around 500 nanometers)are seen to be reduced in comparison to the side lobes shown in FIG. 16,while the zero-order resonance remains at high efficiency. The bandwidthof the zero-order is seen to be smaller than for the four-level caseillustrated in FIG. 16 due to the larger number of steps used for thegrating used in relation to FIG. 17. The graph of FIG. 17 can becompared with the graph of FIG. 10B (showing the diffraction efficiencyfor a one-dimensional totally asymmetric ten-level stepped grating). Thegrating corresponding to FIGS. 16 and 17 may be considered to be extremecases, in that the uniformity of the grating profiles in the y-directionis altered to such an extreme degree.

Referring to FIG. 18, an isometric view of a four-level asymmetriccrossed grating that is stepped in both the x and y directions is shown.This four-level grating structure may be similar to the structure thatwas used for the diffraction efficiency computation of FIG. 16. Incontrast, for the one-dimensional gratings that were used for thecomputations shown in FIGS. 9-13, the step variation was only in thex-direction.

The foregoing description has analyzed surface relief steppedstructures, using the full vector theory, and has produced spectralscans covering the diffraction efficiencies in reflection from the nearultraviolet, through the visible region, to the near infrared. Theanalysis has considered the general case of surface relief diffractiongratings, with variations in the step heights, depth, gratingperiodicity, and symmetry of the grating profiles. The analysis alsoconsidered crossed-grating two dimensional profiles. For certaincombinations of these variables it was shown that the diffracted lightcan exhibit narrow band and broadband single primary colors, such asred, green, or blue, or apparent single colors, such as cyan, yellow, ormagenta. In the case where a certain narrow portion of the spectrum isdelineated by an envelope of several closely spaced narrowband singlecolors, the diffracted light is brighter than for the corresponding caseof a standard volume hologram, for which only a single resonance isvisible. Furthermore, deep focus of an image is maintained, becausefocusing depends on the narrowness of each separate resonance.

The theoretical considerations discussed herein apply to any surfacerelief periodic structure, regardless of the method of formation. Theemphasis in this disclosure has been on holography as the preferredmethod, but other techniques would be equally valid, ranging fromelectron-beam writing to the use of reactive ion etching and ion beammilling to deposition of alternate dielectric layers by vacuumevaporation, to name only a few.

The holographic techniques discussed here have shown that surface reliefstepped structures can be obtained that produce a full diffractivespectral response in reflection throughout the entire visible spectrum.Such techniques include prism coupling and the use of intermediatestandard volume holograms. In particular, the method combinesinterference of object light with two reference beams of light, one forsurface interference patterns that lead to deep etching of photoresist,and the second for volume interference patterns that lead to welldefined steps to produce single color effects. This combinationadditionally produces the special effects of, for example, multipleresonances that lead to high brightness and potentially large variationsin color.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method for recording a holographic image of anobject, the method comprising: providing a recording medium configuredto record a holographic image, the recording medium having two principalsurfaces; placing the object proximate one of the two principal surfacesof the recording medium; and directing radiation beams at anothersurface of the two principal surfaces of the recording medium at anangle with respect to a normal to the other of the two principalsurfaces of the recording medium such that a resultant radiation beamreflected from the object interferes with a resultant first referencebeam progressing in a direction that is at least partly similar to aprogression direction of the resultant radiation beam reflected from theobject, both beams being separated by a small angle such that a firstinterference fringe structure is formed in the recording medium that isskewed with respect to any of the two principal surfaces of therecording medium, and such that a resultant second reference beamprogresses in a general direction that is at least partly opposite tothe progression direction of the resultant radiation beam reflected fromthe object such that a second, fine-sized, set of interference fringesis formed in the recording medium, the second set of interferencefringes being skewed with respect to the any of the two principalsurfaces of the recording medium.
 2. The method of claim 1 wherein theskewed first interference fringe structure includes at least one of: aninclined first interference fringe structure and a non-symmetricinterference fringe structure.
 3. The method of claim 1 whereinproviding the recording medium comprises: providing a photoresistmedium.
 4. The method of claim 1 wherein placing the object comprises:encapsulating the object in an optically clear material; and placing theencapsulated object proximate the one of the two principal surfaces ofthe recording medium.
 5. The method of claim 4 wherein encapsulating theobject in the optically clear material comprises encapsulating theobject in an optically clear cement index matched to a cover glass. 6.The method of claim 1 wherein placing the object comprises: placing aconventional volume hologram of a tangible object proximate the one ofthe two principal surfaces of the recording medium.
 7. The method ofclaim 6 wherein the conventional volume hologram is encapsulated betweenflat glass plates, and wherein the conventional volume hologram isrecorded in one of dichromated gelatin (DCG), photopolymer and silverhalide.
 8. The method of claim 6 wherein placing the conventional volumehologram comprises: placing the conventional volume hologram at an anglewith respect to any of the two principal surfaces of the recordingmedium.
 9. The method of claim 1 further comprising: reproducingholographic images of the object based, at least in part, on the firstand second interference structures formed in the recording medium. 10.The method of claim 1 wherein providing the recording medium comprises:providing a recording medium configured to be structurally modified whenexposed to interfering and non-interfering portions of radiation beams,the structurally modified recording medium including, when viewed in atwo-dimensional cross-section along one of the axes of the recordingmedium: a plurality of equally spaced steps of fine-sized periodicitysuperimposed upon a plurality of deep depressions of substantiallycoarse-sized periodicity, wherein the structurally modified recordingmedium is configured to produce in reflection single and multiple colorsin a broad spectral range when illuminated by a source of light.
 11. Asystem to record a holographic image, the system comprising: a recordingmedium having two principal surfaces and configured to recordholographic image of an object placed proximate one of the two principalsurfaces of the recording medium; and a radiation source to directradiation beams at another surface of the two principal surfaces of therecording medium at an angle with respect to a normal to the other ofthe two principal surfaces of the recording medium such that a resultantradiation beam reflected from the object interfere with a resultantfirst reference beam progressing in a direction that at least partly issimilar to a progression direction of the resultant radiation beamreflected from the object, both beams being separated by a small anglesuch that a first interference fringe structure is formed in therecording medium that is inclined at an angle with respect to any of thetwo principal surfaces of the recording medium, and such that aresultant second reference beam progresses in a general direction thatat least partly is opposite to the progression direction of theresultant radiation beam reflected from the object such that a second,fine-sized, set of interference fringes is formed in the recordingmedium, the second set of interference fringes being inclined withrespect to the any of the two principal surfaces of the recordingmedium.
 12. The system of claim 11 wherein the radiation source is alaser light source.